The present invention relates to a magnetic recording medium, a production method thereof, and a magnetic recording device. More specifically, the present invention relates to a magnetic recording medium comprising a ferromagnetic metal layer of magnetic crystal grains which are not susceptible to the effects of thermal agitation, as well as a production method thereof, and a magnetic recording device. The magnetic recording medium of the present invention can be ideally applied to hard disks, floppy disks, and magnetic tapes and the like.
In recent years, magnetic recording media have been widely used as high density, large capacity recording media in magnetic recording devices such as hard disks, although improvements are now required in recording and playback characteristics in order to progress to even higher recording densities.
FIG. 13 is a schematic cross-sectional view showing a magnetic recording medium 50 used in a conventional magnetic recording device. Furthermore, FIGS. 14A-D are diagrams showing go each of the steps in a production method for a conventional magnetic recording medium, and a cross-sectional view of the sample at each of those steps. A base material 51 of a non-magnetic material can utilize, for example, a non-magnetic layer 53 comprising nickel-phosphorus (Nixe2x80x94P) provided on the surface of a base 52 comprising an aluminum (Al) based alloy. An underlayer 54 comprising a non-magnetic metallic film such as chromium (Cr) for example, a recording layer 55 comprising a magnetic film, and a protective layer 56 are then layered sequentially on top of the base material 51.
In a medium comprising a Co based magnetic layer 55 laminated on top of a Cr underlayer 54, the following facts are well known.
(1) The thinner the Cr layer 54 becomes, the smaller the grain diameter of the crystal grains 54a which make up the Cr layer 54 will be, and the thicker the Cr layer 54 becomes, the larger the grain diameter of the crystal grains 54a will become.
(2) The grain diameter of the crystal grains 55a which make up the Co based magnetic layer 55 laminated on top of the Cr underlayer 54 will be of approximately the same size as the grain diameter of the crystal grains 54a of the Cr layer 54 [FIG. 14(d)].
In contrast, in order to improve the recording and playback characteristics of a magnetic recording medium 50 of the construction described above, the present inventor and others have already reported [M. Takahashi, A. Kikuchi and S. Kawakita: IEEE Trans. on Magn., 33, 2938 (1997)] that a reduction in the interaction between grains of the magnetic crystal grains 55a which make up the magnetic film which functions as the recording layer 55, and a reduction in the film thickness of the magnetic film, are essential. In particular, in order to achieve a reduction in noise level for the medium, the above reference introduces a fabrication method effective for miniaturizing the crystal grains 55a of the magnetic film, by reducing the film thickness of the magnetic film of the recording layer 55.
However, there are limits to the micro-structure control and volume reduction in the crystal grains 55a which are possible by reduction in the thickness of the magnetic film comprising the recording layer 55. The reason being that as the thickness of the magnetic film comprising the recording layer 55 is reduced, there is an accompanying miniaturization of the crystal grains 55a which make up the magnetic film, and a problem arises in that the magnetic characteristics, such as the magnetization (residual magnetization) recorded on the magnetic film, can vary significantly over time, in other words the magnetic film becomes more susceptible to thermal agitation.
The present inventor has keenly pursued the development of a method for reducing the grain diameter of the crystal grains 55a of a magnetic film by reducing the film thickness of the underlayer 54, in other words, a method of reducing the volume of the crystal grains 55a of the magnetic film. Specifically, by developing a method of preparing a medium comprising a Cr underlayer and a Co based magnetic layer under ultra clean process conditions, the inventor has succeeded in developing a medium capable of achieving a coercive force exceeding 2000 [Oe] even with an ultra-thin Cr layer with a thickness of 2.5 nm as the underlayer 54 (International Patent Application No. PCT/JP97/01092).
The ultra clean process disclosed in the aforementioned application is a process comprising principally an increase in the ultimate vacuum of the film formation chamber from the 10xe2x88x927 Torr level of a typical conventional sputtering device, to the 10xe2x88x929 Torr level, as well as a reduction in the concentration of impurities such as water in the ultra pure Ar gas introduced into the film formation chamber by a further two digits beyond the levels found in normal ultra pure Ar gas, down to the 1 ppb level.
According to the aforementioned method, the average grain diameter of the crystal grains 55a making up the magnetic film decreased, although there was a tendency for the variation (standard deviation) in size to increase. This effect reflects localized variations in the interactions between grains within the magnetic film, and as the shift to higher recording densities proceeds, and recording magnetization becomes smaller and smaller, the above effect can no longer be ignored.
Furthermore, the effects achieved by reducing the film thickness of the underlayer using the aforementioned medium production method are approaching a limit, and the tendency for the interaction between grains of the magnetic film to increase with reductions in the film thickness of the underlayer has also been confirmed for specific magnetic films [J. Nakai, A. Kikuchi, K. Nakatani, M. Hirasaka, T. Shimatsu and M. Takahashi: J. Magn. Magn. Mater., 155, 234 (1996)].
Accordingly, there has been considerable demand for the development of a magnetic recording medium which enables a reduction in the grain diameter of the magnetic crystal grains 55a using a method other than reducing the film thickness of the underlayer 54 or the recording layer 55, and also enables the suppression of variation in the grain diameter of the magnetic crystal grains 55a, as well as a production method for such a medium, and a magnetic recording device.
One object of the present invention is to provide a magnetic recording medium capable of suppressing the effects of thermal agitation by simultaneously reducing the average grain diameter and the standard deviation of the magnetic crystal grains of a ferromagnetic metal layer, without changing the film thickness of a metal underlayer or the film thickness of the ferromagnetic metal layer which forms the recording layer.
Furthermore another object of the present invention is to provide a method of producing a magnetic recording medium capable of suppressing the effects of thermal agitation by simultaneously reducing the average grain diameter and the standard deviation of the magnetic crystal grains of a ferromagnetic metal layer without changing the film thickness of a metal underlayer or the film thickness of the ferromagnetic metal layer which forms the recording layer.
In addition, yet another object of the present invention is to provide a magnetic recording device comprising a magnetic recording medium capable of suppressing the effects of thermal agitation by simultaneously reducing the average grain diameter and the standard deviation of the magnetic crystal grains of a ferromagnetic metal layer without changing the film thickness of a metal underlayer or the film thickness of the ferromagnetic metal layer which forms the recording layer.
A magnetic recording medium according to the present invention is a magnetic recording medium comprising a ferromagnetic metal layer of a cobalt based alloy formed on a base material with a metal underlayer comprising chromium as a major constituent disposed there between, wherein a seed layer comprising at least tungsten is provided between the base material and the metal underlayer, and this seed layer is an islands type film.
Furthermore, in a magnetic recording medium of the above construction, the seed layer may also comprise chromium, in addition to tungsten.
Moreover, in a magnetic recording medium of the above construction, the aforementioned base material may comprise a non-magnetic base and a coating film, wherein the coating film can effectively utilize an alloy comprising mainly nickel and an element which is capable of co-precipitated with nickel and which has a strong affinity for oxygen.
The aforementioned element which is capable of co-precipitated with nickel and which has a strong affinity for oxygen can be either one, or two or more elements selected from the group consisting of phosphorus, cobalt, tungsten, iron, vanadium, chromium, manganese, copper, zinc, molybdenum, palladium, tin, rhenium, aluminum, zirconium, boron, titanium, hafnium, niobium and tantalum.
A production method for a magnetic recording medium according to the present invention is a production method for a magnetic recording medium comprising a seed layer comprising at least tungsten, a metal underlayer comprising chromium as a major constituent, and a ferromagnetic metal layer of a cobalt based alloy formed sequentially on top of a base material; comprising at least a preliminary processing step for positioning the base material inside a film formation chamber, evacuating the inside of the film formation chamber down to a predetermined degree of vacuum and then heating the base material to a predetermined temperature; an intermediate processing step in which a process D for dry etching the base material, and a process S for depositing a seed layer comprising at least tungsten in an islands type pattern onto the base material, are performed at least once under a pressure environment greater than the aforementioned predetermined degree of vacuum; and a post processing step for sequentially depositing the aforementioned metal underlayer and the aforementioned ferromagnetic metal layer on top of the seed layer.
Furthermore, in a production method for a magnetic recording medium of the above construction, the aforementioned intermediate processing step may also comprise, in addition to the aforementioned process D and the aforementioned process S, a process O which is performed at least once, in which the aforementioned base material is exposed to a predetermined oxygen atmosphere under a pressure environment greater than the aforementioned predetermined degree of vacuum.
A magnetic recording device according to the present invention comprises a magnetic recording medium of the above construction, a drive section for driving the aforementioned magnetic recording medium, a magnetic head, and a movement device for moving the magnetic head relative to the magnetic recording medium.